Shallow trench isolation structure with raised portion between active areas and manufacturing method thereof

ABSTRACT

A semiconductor structure includes a semiconductor substrate, a first active area, a second active area, a first trench, at least one raised portion, and a first dielectric. The first active area is in the semiconductor substrate. The second active area is in the semiconductor substrate. The first trench is in the semiconductor substrate and separates the first active area and the second active area from each other. The raised portion is raised from the semiconductor substrate and is disposed in the first trench. The first dielectric is in the first trench and covers the raised portion.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims priority to U.S. Provisional Application Ser. No. 62/098,101, filed Dec. 30, 2014, which is herein incorporated by reference.

BACKGROUND

Shallow trench isolation (STI) is an integrated circuit feature which prevents electrical current leakage between adjacent semiconductor devices. STI is generally used on complementary metal-oxide-semiconductor (CMOS) process technology nodes of 250 nanometers and smaller. Older CMOS technologies and non-metal-oxide-semiconductor (MOS) technologies commonly use isolation based on local oxidation of silicon (LOCOS).

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1-8 are cross-sectional views of shallow trench isolation (STI) structures at various stages in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

The singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising”, or “includes” and/or “including” or “has” and/or “having” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.

It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by the person having ordinary skill in the art. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

FIGS. 1-8 are cross-sectional views of shallow trench isolation (STI) structures at various stages in accordance with some embodiments.

Reference is made to FIG. 1. A hard mask layer 110 is formed on a substrate 120 and has openings 112 therein to define active areas and raised portions which will be formed in the followings steps. The substrate 120 is made of a semiconductor material, such as diamond, silicon, germanium, silicon carbide, silicon-germanium, or combinations thereof. The substrate 120 is, for example, doped or undoped bulk silicon or an active layer of a silicon on insulator (SOI) substrate. Other substrates that may be used include multi-layered substrates, gradient substrates, or hybrid orientation substrates.

The active areas will be used for components of active devices, such as n-channel metal-oxide-semiconductor field-effect transistors (n-channel MOSFETs), p-channel MOSFETs, planar MOSFETs, or fin field-effect transistors (finFETs), to be formed later. The active areas are formed by, for example, the implantation of dopants into the substrate 120. Depending upon the dopants chosen, the active areas may include n-wells or p-wells as determined by the design. If the substrate 120 is made of a Group IV semiconductor material, such as diamond, silicon, germanium, silicon carbide, silicon-germanium, or combinations thereof, the dopants can be acceptors from Group III or donors from Group V elements. For example, boron (B), aluminium (Al), indium (In), gallium (Ga), or combinations thereof, having three valence electrons, can be used as the dopants to form a p-well in the substrate 120 when the substrate 120 is made of a Group IV semiconductor material with four valence electrons. On the other hand, phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi), or combinations thereof, having five valence electrons, can be used as the dopants to form an n-well in the substrate 120 when the substrate 120 is made of a Group IV semiconductor material with four valence electrons.

The hard mask layer 110 is made of a material which can be a barrier against water molecules and oxygen. In some embodiments, the hard mask layer 110 is made of, for example, silicon nitride. The hard mask layer 110 has a thickness in a range from about 400 angstroms to about 2000 angstroms. The hard mask layer 110 is formed by, for example, chemical vapor deposition (CVD), low pressure chemical vapor deposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD), or other deposition processes.

The terms “about” may be applied to modify any quantitative representation which could permissibly vary without resulting in a change in the basic function to which it is related. For example, the hard mask layer 110 as disclosed herein having a thickness in a range from about 400 angstroms to about 2000 angstroms may permissibly have a thickness somewhat less than 400 angstroms if its barrier capability is not materially altered.

A barrier layer 115 may be formed on the substrate 120 before the hard mask layer 110 is formed. The barrier layer 115 is made of a material which can enhance an adhesion between the hard mask layer 110 and the substrate 120. In some embodiments, the barrier layer 115 is made of, for example, silicon oxide. The barrier layer 115 has a thickness in a range from about 50 angstroms to about 200 angstroms. The barrier layer 115 is formed by a thermal process, such as thermal oxidation or other deposition processes.

Although FIG. 1 shows the barrier layer 115 is sandwiched between the hard mask layer 110 and the substrate 120, the barrier layer 115 could permissibly be omitted. In some embodiments, the hard mask layer 110 can be formed on the substrate 120 in absence of the barrier layer 115 if the adhesion between the hard mask layer 110 and the substrate 120 is at an acceptable level.

The hard mask layer 110 and the barrier layer 115 are patterned to form the openings 112 therein to expose portions of the substrate 120 where trenches will be formed in the followings steps. The hard mask layer 110 and the barrier layer 115 are patterned by a photolithography and etching process. The photolithography and etching process includes photoresist application, exposure, developing, etching, and photoresist removal. The photoresist is applied onto the substrate 120 by, for example, spin coating. The photoresist is then prebaked to drive off excess photoresist solvent. After prebaking, the photoresist is exposed to a pattern of intense light. The exposure to light causes a chemical change that allows some of the photoresist soluble in a photographic developer. A post-exposure bake (PEB) may be performed before developing to help reduce standing wave phenomena caused by the destructive and constructive interference patterns of the incident light. The photographic developer is then applied onto the photoresist to remove the some of the photoresist soluble in the photographic developer. The remaining photoresist is then hard-baked to solidify the remaining photoresist. Portions of the hard mask layer 110 and the barrier layer 115 which are not protected by the remaining photoresist are etched to form the openings 112. The etching of the hard mask layer 110 and the barrier layer 115 may be, for example, reactive-ion etching (RIE). After etching the hard mask layer 110 and the barrier layer 115, the photoresist is removed from the substrate 120 by, for example, ashing or stripping.

The Reactive-ion etching (RIE) is a type of dry etching which has different characteristics than wet etching. RIE uses chemically reactive plasma to form the openings 112. The plasma is generated under low pressure (vacuum) by an electromagnetic field. High-energy ions from the chemically reactive plasma attack the hard mask layer 110 and the barrier layer 115 and react with them. In some embodiments, fluorocarbon or hydrofluorocarbon based RIE can be used to form the openings 112.

A cleaning process may be performed to remove a native oxide of the substrate 120 after the hard mask layer 110 and the barrier layer 115 are patterned. In some embodiments, the native oxide of the substrate 120 can be removed by hydrofluoric acid. The cleaning process is optional. In some embodiments, the cleaning process can be omitted if the native oxide of the substrate 120 is at an acceptable level.

Reference is made to FIG. 2. Trenches 122 are formed in the substrate 120. The trenches 122 define the active areas 124 and the raised portions 126. That is, the trenches 122 separate the active areas 124 and the raised portions 126 from one another. The exposed portions of the substrate 120 through the openings 112 are removed by an etching process, such as reactive-ion etching (RIE), in order to form the trenches 122 in the substrate 120.

In some embodiments, chlorine (Cl) or bromine (Br) based reactive-ion etching (RIE) can be used to form the trenches 122. At least one of the trenches 122 has a depth in a range from about 0.3 μm to about 0.5 μm. At least one of the trenches 122 has at least one sidewall S, a bottom surface B, and a taper angle α between the sidewall S and a plane extending from the bottom surface B. The taper angle α of the trench 122 is in a range from about 78° to about 88°.

Reference is made to FIG. 3. The heights of the raised portions 126 are reduced by, for example, a photolithography and etching process. After reducing the heights of the raised portions 126, the trenches 122 defining the raised portions 126 are combined into a first trench 122 a, and the raised portions 126 are disposed in the first trench 122 a. The photolithography and etching process includes photoresist application, exposure, developing, etching, and photoresist removal. The photoresist is applied onto the substrate 120 by, for example, spin coating. The photoresist is then prebaked to drive off excess photoresist solvent. After prebaking, the photoresist is exposed to a pattern of intense light. The exposure to light causes a chemical change that allows some of the photoresist soluble in a photographic developer. A post-exposure bake (PEB) may be performed before developing to help reduce standing wave phenomena caused by the destructive and constructive interference patterns of the incident light. The photographic developer is then applied onto the photoresist to remove the some of the photoresist soluble in the photographic developer. The remaining photoresist is then hard-baked to solidify the remaining photoresist. After developing, the raised portions 126 are not protected by the remaining photoresist while the active areas 124 are protected by the remaining photoresist. The raised portions 126, which are not protected by the remaining photoresist, are etched to reduced the heights of the raised portions 126. After etching the raised portions 126, the photoresist is removed from the substrate 120 by, for example, ashing or stripping.

Reference is made to FIG. 4. A liner layer 130 is formed on the sidewalls S and the bottom surfaces B of the trenches 122. In some embodiments, the liner layer 130 is made of, for example, silicon oxide. The liner layer 130 has a thickness in a range from about 100 angstroms to about 400 angstroms. The liner layer 130 may be formed by, for example, thermal oxidation. In some embodiments, a temperature for the formation of the liner layer 130 is in a rage from about 800° C. to about 1200° C.

The liner layer 130 can repair structural damages incurred in the substrate 120 by the etching process for forming the trenches 122. Furthermore, corner rounding may be achieved during the formation of the liner layer 130. During the thermal oxidation, the corners of the substrate 120 at perimeters of the trenches 122 may be rounded automatically as the liner layer 130 is formed because the oxidation speed at the corners of the substrate 120 is higher than the oxidation speed at other surfaces. With the rounded corner, the divot which may be formed in the following steps and may result in a parasitic transistor with a low threshold voltage can be eliminated or reduced to an acceptable level. In some embodiments, the rounded corner has a radius of curvature in a range from about 300 angstroms to about 600 angstroms.

Reference is made to FIG. 5. A flowable dielectric 140 overfills the trenches 122 to form the shallow trench isolation (STI) structures. The flowable dielectric 140 may include a flowable silicon oxide or silicon nitride dielectric material. The flowable dielectric 140 is formed by using a spin on dielectric (SOD), such as a silicate, a siloxane, a methyl SilsesQuioxane (MSQ), a hydrogen SisesQuioxane (HSQ), an MSQ/HSQ, a perhydrosilazane (TCPS), or a perhydro-polysilazane (PSZ). Alternatively, the flowable dielectric 140 may be formed by using a deposition technique, such as plasma enhanced chemical vapor deposition (PECVD). A temperature for the formation of the flowable dielectric 140 in the PECVD is less than about 100° C. A pressure for the formation of the flowable dielectric 140 in the PECVD is in a range from about 100 mTorr to about 10 Torr. A reaction source uses a gaseous environment containing Si₃H₉N and NH₃. In some embodiments, a flow rate of Si₃H₉N is in a range from about 100 standard cubic centimeters per minute (sccm) to about 1000 sccm, and a flow rate of NH₃ is in a range from about 100 sccm to about 2000 sccm.

Reference is made to FIG. 6. A curing process is performed on the flowable dielectric 140. In some embodiments, the curing process is operated in a flow rate of O₃ in a range from about 100 standard cubic centimeters per minute (sccm) to about 5000 sccm. A temperature for the curing process is in a range from about 10° C. to about 500° C. A pressure for the curing process is in a range from about 1 Torr to about 760 Torr. It is believed that the curing of the flowable dielectric 140 enables the transformation of Si—O bond network to densify the flowable dielectric 140.

An anneal process is then performed. The anneal process could further densify and improve the quality of the flowable dielectric 140. In some embodiments, the anneal process is performed in an environment containing steam. A flow rate of the steam is in a range from about 5 standard cubic centimeters per minute (sccm) to about 20 sccm. A temperature for the anneal process is in a range from about 1000° C. to about 1200° C. The anneal process may start at about 200° C. and ramp up the temperature gradually to a predetermined temperature of about 1000° C. to about 1200° C.

During the curing and/or the annealing of the flowable dielectric 140, the flowable dielectric 140 shrinks as it densifies. The shrinkage of the flowable dielectric 140 applies tensile force on the active areas 124 and the raised portions 126. The tensile force applied on the active areas 124 and the raised portions 126 depends on the amount that the flowable dielectric 140 shrinks. That is, the tensile force applied on the active areas 124 and the raised portions 126 increases as the amount that the flowable dielectric 140 shrinks increases. Therefore, the flowable dielectric 140 in the larger trench 122 may apply greater tensile force on the active areas 124 or the raised portions 126.

For example, the active areas 124 of FIG. 6 include a first active area 124 a, a second active area 124 b, and a third active area 124 c. The trenches 122 include the first trench 122 a and a second trench 122 b. The flowable dielectric 140 includes a first dielectric 140 a and a second dielectric 140 b. The first trench 122 a separates the first active area 124 a and the second active area 124 b from each other. The second trench 122 b separates the first active area 124 a and the third active area 124 c from each other. The first dielectric 140 a is in the first trench 122 a. The second dielectric 140 b is in the second trench 122 b. The first trench 122 a is wider than the second trench 122 b. That is, a first distance D1 between the first active area 124 a and the second active area 124 b is greater than a second distance D2 between the first active area 124 a and the third active area 124 c. Since the first dielectric 140 a and the second dielectric 140 b are made of substantially the same flowable dielectric material, the tensile force due to the shrinkages of the first dielectric 140 a and the second dielectric 140 b mainly depends on the sizes of the first trench 122 a and the second trench 122 b. Therefore, if the raised portions 126 were absent from the first trench 122 a, the first dielectric 140 a would apply greater tensile force on the first active area 124 a than the tensile force applied by the second dielectric 140 b on the first active area 124 a. The unbalance tensile force applied on the first active area 124 a may bend the first active area 124 a and thus damage the first active area 124 a or introduce crystal defects in the first active area 124 a.

As shown in FIG. 6, the raised portions 126 are formed in the first trench 122 a to reduce the shrinkage of the first dielectric 140 a. With the raised portions 126, the tensile force applied on the first active area 124 a by the first dielectric 140 a and the second dielectric 140 b is substantially balanced. Therefore, the damage to the first active area 124 a or the crystal defects introduced in the first active area 124 a by the unbalanced tensile force can be eliminated or reduced to an acceptable level.

In some embodiments, the tensile force due to the shrinkage of the first dielectric 140 a may be applied on the raised portions 126 to bend the raised portions 126. In those embodiments, the raised portions 126 can be considered sacrificial structures which protect at least the first active area 124 a from being bent by the shrinkage of the first dielectric 140 a.

In some embodiments, the raised portion is formed in a trench which is disposed in an isolation area of the substrate. The trench in the isolation area of the substrate is wider than the trench in a dense area of the substrate. The raised portion is formed in the trench in the isolation area of the substrate to reduce the amount of dielectric shrinkage, thereby balancing the tensile force applied on the active area. For example, the first trench 122 a of FIG. 6 is in the isolation area of the substrate 120, and the second trench 122 b of FIG. 6 is in the dense area of substrate 120. The first trench 122 a is wider than the second trench 122 b. The raised portions 126 are formed in the first trench 122 a to reduce the shrinkage of the first dielectric 140 a, thereby balancing the tensile force applied on the first active area 124 a.

The number of the raised portion(s) in one trench depends on the size of the trench. Specifically, the number of the raised portion(s) increases as the size of the trench increases when the size(s) of the raised portion(s) stays the same.

Although FIG. 6 shows the number of the raised portions 126 in the first trench 122 a is two, the number of the raised portions 126 in the first trench 122 a could permissibly vary without resulting in a material change in the force balance. For example, there may permissibly be other numbers of raised portion(s) in one trench, such as one, three, four, or more, within the claimed scope if the force balance is not materially altered.

The size(s) of the raised portion(s) depends on the size of the trench as well. Specifically, the size(s) of the raised portion(s) increases as the size of the trench increases when the number of the raised portion(s) stays the same.

In some embodiments, at least one of the raised portions 126 has a width in a range from about 50 nm to about 400 nm. At least one of the raised portions 126 has a length in a range from about 50 nm to about 400 nm. At least one of the raised portions 126 has a height in a range from about 50 nm to about 250 nm. The size of the raised portion 126 could permissibly vary without resulting in a material change in the force balance. For example, the height of the raised portion 126 may permissibly be less than 50 nm within the claimed scope if the force balance is not materially altered.

Reference is made to FIG. 7. The excess flowable dielectric 140 outside of the trenches 122 is removed through a removal process. In some embodiments, the flowable dielectric 140 over burden is removed by a chemical mechanical polishing (CMP) process. In some embodiments, a combination of a plasma etch-back followed by the CMP process is used. The hard mask layer 110 acts as a polish stop layer to protect the underlying active areas 124 from CMP damage.

Reference is made to FIG. 8. The hard mask layer 110 and the barrier layer 115 are removed by an etching process, such as a wet etching process. In some embodiments, the hard mask layer 110 can be removed by hot phosphoric acid when the hard mask layer 110 is made of silicon nitride, and the barrier layer 115 can be removed by hydrofluoric acid when the barrier layer 115 is made of silicon oxide. The removals of the hard mask layer 110 and the barrier layer 115 expose the active areas 124 to further process steps.

Reference is made to FIG. 8. The raised portions 126 are buried under the first dielectric 140 a. That is, the first dielectric 140 a covers the raised portions 126. In some embodiments, the first active area 124 a is higher than the raised portions 126, and the second active area 124 b is higher than the raised portions 126 as well. At least one of the raised portions 126 has a top surface 126 t. The first dielectric 140 a has a top surface 140 t. The top surface 126 t of the raised portion 126 is lower than the top surface 140 t of the first dielectric 140 a. Since the raised portions 126 are buried under the first dielectric 140 a, the first dielectric 140 a can still electrically isolate the first active area 124 a and the second active area 124 b from each other even if the raised portions 126 are made of a semiconductor material. In some embodiments, the raised portions 126 and the substrate 120 are made of substantially the same semiconductor material since the raised portions 126 are formed by etching the substrate 120.

In order to prevent the active area from being damaged by the shrinkage of the flowable dielectric, at least one raised portion is formed to balance the tensile force applied on the active area. When a shallow trench isolation (STI) trench adjacent to the active area is too large such that the shrinkage of the flowable dielectric in the STI trench applies unbalance tensile force on the active area, at least one raised portion can be formed in the STI trench to balance the tensile force applied on the active area. The raised portion can be a sacrificial structure which absorbs the tensile force created by the shrinkage of the flowable dielectric and thus protects the active area from being damaged by the shrinkage of the flowable dielectric. Furthermore, the raised portion is buried under the flowable dielectric, and therefore the flowable dielectric can still electrically isolate the active area from other areas even if the raised portion is made of a semiconductor material.

According to some embodiments, a semiconductor structure includes a semiconductor substrate, a first active area, a second active area, a first trench, at least one raised portion, and a first dielectric. The first active area is in the semiconductor substrate. The second active area is in the semiconductor substrate. The first trench is in the semiconductor substrate and separates the first active area and the second active area from each other. The raised portion is raised from the semiconductor substrate and is disposed in the first trench. The first dielectric is in the first trench and covers the raised portion.

According to some embodiments, a semiconductor structure includes a semiconductor substrate, a first dielectric, a second dielectric, and at least one raised portion. The semiconductor substrate has a first trench and a second trench therein. The first trench and the second trench define an active area therebetween. The first dielectric is in the first trench. The second dielectric is in the second trench. The raised portion is raised from the semiconductor substrate. The raised portion is disposed in the first trench and is buried under the first dielectric.

According to some embodiments, a method for forming a semiconductor structure includes the following steps. A first trench and a second trench are formed in a semiconductor substrate to define a raised portion between the first trench and the second trench. A height of the raised portion is reduced, such that the first trench and the second trench are combined to form a shallow trench isolation (STI) trench. The STI trench is filled with a flowable dielectric. The flowable dielectric is cured.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A semiconductor structure comprising: a semiconductor substrate; a first active area in the semiconductor substrate; a second active area in the semiconductor substrate; a first trench in the semiconductor substrate and separating the first active area and the second active area from each other; at least one raised portion raised from the semiconductor substrate and disposed in the first trench, the raised portion having opposite first and second sidewalls, wherein an angle of incline of the second sidewall is greater than an angle of incline of the first sidewall; and a first dielectric in the first trench and covering the raised portion.
 2. The semiconductor structure of claim 1, wherein the first dielectric is made of a cured flowable dielectric material.
 3. The semiconductor structure of claim 1, wherein the first active area is higher than the raised portion.
 4. The semiconductor structure of claim 3, wherein the second active area is higher than the raised portion.
 5. The semiconductor structure of claim 1, wherein the raised portion has a top surface, the first dielectric has a top surface, and the top surface of the raised portion is lower than the top surface of the first dielectric.
 6. The semiconductor structure of claim 1, further comprising: a third active area in the semiconductor substrate; a second trench in the semiconductor substrate and separating the first active area and the third active area from each other, wherein a first distance between the first active area and the second active area is greater than a second distance between the first active area and the third active area; and a second dielectric in the second trench.
 7. The semiconductor structure of claim 6, wherein the first dielectric and the second dielectric are made of substantially the same cured flowable dielectric material.
 8. The semiconductor structure of claim 1, wherein the raised portion and the semiconductor substrate are made of substantially the same semiconductor material.
 9. The semiconductor structure of claim 1, wherein a plurality of the raised portions are disposed in the first trench.
 10. A semiconductor structure comprising: a semiconductor substrate having a first trench and a second trench therein, wherein the first trench and the second trench define an active area therebetween; a first dielectric in the first trench; a second dielectric in the second trench; and at least one raised portion raised from the semiconductor substrate, the raised portion being disposed in the first trench and buried under the first dielectric, the raised portion having opposite first and second sidewalls, wherein the second sidewall is steeper than the first sidewall.
 11. The semiconductor structure of claim 10, wherein the first dielectric is made of a cured flowable dielectric material.
 12. The semiconductor structure of claim 11, wherein the first dielectric and the second dielectric are made of substantially the same cured flowable dielectric material.
 13. The semiconductor structure of claim 10, wherein the first trench is wider than the second trench.
 14. The semiconductor structure of claim 10, wherein the active area is higher than the raised portion.
 15. The semiconductor structure of claim 10, wherein the raised portion has a top surface, the first dielectric has a top surface, and the top surface of the raised portion is lower than the top surface of the first dielectric.
 16. The semiconductor structure of claim 10, wherein the raised portion and the semiconductor substrate are made of substantially the same semiconductor material.
 17. The semiconductor structure of claim 10, wherein a plurality of the raised portions are disposed in the first trench.
 18. A semiconductor structure comprising: a semiconductor substrate having a first trench and a second trench therein; a first active area in the semiconductor substrate and between the first trench and the second trench; a second active area in the semiconductor substrate, wherein the first trench is between the first active area and the second active area; at least one raised portion raised from a bottom of the first trench, wherein the raised portion is tilted towards a center of the first trench; and a first dielectric in the first trench and covering the raised portion.
 19. The semiconductor structure of claim 18, wherein the raised portion has a top surface, the first dielectric has a top surface, and the top surface of the raised portion is lower than the top surface of the first dielectric.
 20. The semiconductor structure of claim 18, further comprising: a second dielectric in the second trench. 